Accelerometers have been employed in a variety of applications. For example, accelerometers have been employed to help determine the acceleration or deceleration of a ship or plane, to monitor the forces being applied to an apparatus or device, such as a car, train, bus, and the like.
A typical prior art accelerometer used a pendulum type transducer in which acceleration is detected by noting the displacement of the pendulum. A force is applied to the pendulum, generally by electromagnetic currents, in order to force the pendulum back to its initial, resting position. By measuring the current required to generate this electromagnetic field, the acceleration can be determined. From there, the product of the acceleration times the mass is the force.
More modern accelerometers rely on a movable electrode located between two fixed electrodes, as discussed in Suzuki and Tuchitani, "Semiconductor Capacitance-type Accelerometer with PWM Electrostatic Servo Technique," Sensors and Actuators, A1-A23 (1990) 316-319, and European Patent Application No. EPO 338688 A1. The Suzuki, et al. invention employs a silicon movable electrode located at the end of a cantilever attached to a silicon base. The movable electrode is spaced apart from two fixed electrodes located on either side of the movable electrode. This apparatus is sandwiched within a glass structure and electrically connected to monitoring circuitry. Circuitry is generally shown in the Suzuki article and patent application.
Additional circuit configurations are described in U.S. Pat. No. 5,142,921, issued to Stewart, et al., and U.S. Pat. No. 3,877,313, issued to Ferriss, et al.
U.S. Pat. No. 4,679,434, issued to Stewart, discloses a sandwich-type accelerometer in which a semiconductor substrate is sandwiched between two non-conductive plates. This configuration employs hinges having crossed blades to provide the desired flexure and strength. The accelerometer is kept in close proximity to the signal processing circuitry by mounting it in a hybrid package along with the signal processing circuitry.
Both the Stewart accelerometer and the Suzuki accelerometer require the intricate assembly of a three-piece structure. Proper alignment and orientation of the fixed electrodes with the movable electrode are necessary for proper operation of the device. This alignment and assembly are made more difficult by the physical size of the devices. The movable electrode is necessarily extremely thin and the flexures are also fragile.
Even more delicate is the cantilever of Suzuki or the hinge of Stewart, both of which can be readily fractured or snapped by rough handling. In the cantilever of Suzuki or the hinge of Stewart, it is also more difficult to keep both sides of the middle substrate clean during three-layer alignment, and more difficult to "chuck" or hold the middle wafer of a three-wafer stack.
An even greater danger is the formation of microcracks within the crystalline structure of the cantilever member. These microcracks may go undetected during assembly, but may fail in operation or begin to produce erroneous readings as the cracks propagate and/or reduce electrical conductivity.
The crossed blade design of Stewart provides lateral stability of the movable electrode, allowing the movable electrode to flex along the vertical axis. This reduces the sensitivity to anomalies which may result from torsional or twisting forces. The crossed blades of Stewart employ grooves having sharp ends which terminate into the silicon crystal structure. These sharp terminations provide stress points where microcracks may begin to develop.
The Suzuki design employs a single cantilever located near the middle of the movable electrode, also known as the proofmass. This central point of contact makes the Suzuki device susceptible to torsional instabilities arising from the electrostatic negative spring. This can result in erroneous readings as the movable electrode will appear to be closer to both fixed electrodes as a result of the twisting. For higher ranges, the negative spring rate can easily overcome the torsional spring rate of the hinges. This configuration also provides sites where microcracks can originate, leading to a degradation of the device.
U.S. Pat. No. 5,115,291, issued to Stokes, employs a four-step assembly process in order to locate a movable electrode on a cantilever in between two fixed electrodes. The Stokes invention dopes the movable electrode differently than the cantilever. The Stokes invention also employs cantilevers located on all four sides of the movable electrode in order to maintain the position and orientation of the movable electrode.
The Suzuki and other similar devices use Pyrex outer edges on which the fixed electrodes are mounted. Pyrex glass or other types of glass are not exceptional thermal conductors; they are two orders of magnitude less than silicon. Pyrex can support a temperature gradient through the thickness of the glass, however. The semiconductor material forming the fixed electrodes has thermal conductive characteristics far superior to the Pyrex on which it is mounted. The thermal expansion coefficient mismatch between silicon and Pyrex is approximately 10%. This difference is sufficient to cause stress of the silicon from the Pyrex and distortions of the structure as temperature varies.
A need therefore exists for fabricating a solid state measurement instrument such as an accelerometer which can be reliably manufactured with a minimum of handling. The design and structure of the accelerometer should be such that the exposed stress points are reduced, preventing or eliminating the formation of microcracks.